Every release has speed improvements. Yet, perhaps I am imagining things, but my browser experience today is generally slower than the days of Firefox 1.0

What does Firefox 3.6 do compared to Firefox 1.0:
- Phishing and malware Protection: more memory and about 30MB of storage
- Session restore: more memory and about 1MB of storage
- Spellcheck: practically free
- better addon manager: practically free
- better web support including things like SVG: practically free
- awesomebar: more memory and about 5MB of storage

These are a few of the new features that were added to Firefox after version 1.0. Firefox uses more memory because it does more but it certainly isn´t bogged down. My guess is that you mistake the dramatic increase of website complexity with Firefox bloat. If I load Gmail, Greader and Netvibes I am already in the 250+MB range and the loading of those pages reaches 10 seconds after a reboot. This of course also happens with Google Chrome and Opera.